In a communication system such as Cellular or PCS (Personal Communications Services) systems, signals are transmitted between base stations and subscribers using an RF (radio frequency) carrier. There are many propagation effects that will affect such signals in urban, suburban and rural environments, and these include multipath and log normal fading mechanisms. There are also many methods to improve the quality of a received signal, including the use of high gain antennas, handoff between cells (macro-diversity), and diversity reception with two or more antennas at a base station or subscriber (micro-diversity). Generally, handoff between cells produces improvement to log normal or shadow fading, i.e. that fading produced by signal interactions from obstructions or blockages that reduce the effective coverage of the cell.
Multipath or fast fading is produced by the random scattering of radio signals in a cluttered environment in which the signals reflecting and diffracting from a variety of surfaces relatively near the receiver combine at the antenna of the receiver forming a Rayleigh or Rician signal fading profile. Generally, to combat multipath, two antennas are used, spaced apart enough to obtain nearly independent samples of the multipath faded signals (e.g., signals 10 and 11 of FIG. 1) and these signals are applied to a diversity receiver. Common types of diversity include switched, selection, equal gain, and max ratio combining, and generally require two or more antennas which are separated in space or polarization.
One goal of using diversity receive antennas is to reduce the degrading effect of the large variations in signal strength characteristic of multipath fading by selecting the best antenna for reception, until another antenna becomes better. The effect of this is that the active antenna (i.e., currently in use) will be continually changing since the motion of the user or the environment will cause the absolute signal level on each antenna to change, e.g. as shown by signals 10 and 11 in FIG. 1. Both the motion of the subscriber and the motion of the environment (i.e. vehicles and pedestrians in motion, trees moving in the wind, etc.) will produce variations in the instantaneous signals seen at each antenna. In fact, a user might be stationary, and the signal could still exhibit dynamic multipath fading due to the motion around the user. Environmentally induced fluctuations in the multipath signal can generally be considered to have the same effect as signal fluctuations caused by the subscriber's movement since approximately the same receiver degradation due to signal fading would occur.
While the use of diversity does improve the quality of signal reception, it accomplishes this improvement using signal strength information, but typically not any specific information about the characteristics of the fading channel and environment. Some prior approaches have attempted to derive such information, but these have been measurements off a single branch (e.g., counting zero crossings and using slope detection) with some degree of complexity in additional circuitry being required. Such determinations have been limited in their quality, and subject to inaccuracies (e.g., when a branch goes into a deep fade).
Finally, the beneficial effects of diversity will be at their maximum when there is no branch imbalance between the paths to the two antennas. This means that when each antenna receives the same average power, as averaged over approximately 20 wavelengths in a scattered environment, the branches are said to be balanced. When the two paths or branches are not balanced, i.e. if there is extra attenuation in one path due to an obstruction, or blockage, or due to polarization effects, etc., then the benefit from diversity is reduced. This is illustrated by FIG. 2, where signal 20 is received with substantial attenuation with respect to signal 21.
A need, therefore, remains for an improved method for estimating characteristics of a fading signal, including those indicative of subscriber speed, while minimizing system complexity and cost, and preferably considering branch imbalances and other known factors.